Electrical resistance device and method of manufacture thereof



- w issrrfrri u (JHOGS nu LI\L-IJSL March 31, 1942. H. L. B. GOULD ETAL2,278,072

ELECTRIbAL RESISTANCE DEVICE AND METHOD OF MANUFACTURE THEREOF FiledJune 5, 1939 I II . H.L.8.GOULD BAKINGSBURY INVENTORS A TTORNEV PatentedMar. 31, 1942 ELECTRICAL RESISTANCE DEVICE AND METHOD OF MANUFACTURETHEREOF Harold L. B. Gould,

Towaco, and Burton A.

Kingsbury, East Orange, N. .L, assignors to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationJune 3, 1939, Serial No. 277,126

7 Claims.

This invention relates to electrical circuit elements the resistance ofwhich varies markedly with temperature. More particularly, thisinvention relates to resistance devices having a high positivetemperature coemcient of resistance, and to methods of manufacturingsuch de-v vices.

Resistance devices in which theqzes'istance is highly sensitive totemperature are used as regulating and control elements in electriccircuits. The so-called semiconductors, because of their highresistance-temperature coefficient and intermediate specific resistancevalues, have been employed considerably in this field. Theresistance-temperature coefiicient of semiconductive materials andresistors made therefrom have heretofore been found to be negative atordinary temperatures.

The positive resistance-temperature coefficient characteristic ofmaterials ordinarily classed as conductors is so small that suchmaterials are inadequate for resistor units where a wide range ofresistance variation is desired. For example, iron does not double inresistance for less than 70 C. rise in temperature at any point below500 C.

One object of this invention is to produce resistance devices having ahigh positive temperature coemcient at relatively low temperatures, forexample, temperatures between and 100 C.

Another object of this invention is to obtain firm, uniform lowresistance contact between the body of a resistor unit and electrodestherefor.

A further object of this invention is to assure uniform spacing of theelectrodes in formed resistance devices.

Still another object of this invention is to increase the density andruggedness of formed electrical resistance devices.

In one illustrative embodiment of this invention, an electricalresistance device comprises a body of a material having a positivetemperature coeflicient of resistance at relatively low temperatures.and electrodes attached to spaced portions of the body. I

In accordance with one feature of this invention. the body of theresistance device is formed of an oxide of chromium, more particularlychromic oxide.

In accordance with another feature of this invention, the electrodes aremade of metallic sheets or gauze intimately adhered to the body of theresistance device.

In accordance with a further feature of this invention, insulatingspacers, such as glass rods,

are incorporated in the body of the resistance device to assure uniformspacing of the electrodes.

In accordance with still another feature of this invention, the formedresistance device is o subjected to heat treatment to produce a denserugged unit.

Other and further objects and features of the invention will beunderstood more fully and clearly from the following detaileddescription with reference to the accompanying drawing in which:

Fig. 1 is a plan view of an illustrative form of resistor unit embodyingthe invention, with parts broken away to show internal structure;

Fig. 2 is a section taken on line 2-2 of Fig. l; and

Figs. 3, 4, 5 and 6 are sectional views of other forms of resistor unitsillustrative of the invention.

Referring now to Figs. 1 and 2 of the drawing, I ll denotes a body ofhigh positive resistancetemperature coeflicient material having elec--trodes H attached thereto. Members of insulating material, such as glassrods l2, may be disposed at intervals throughout the body III to insureuniform spacing of the electrodes. Circuit connections may be made byconductors l3 which are secured to the electrodes II by welding or othersuitable means. The body It comprises finely divided, pressed, chromicoxide or chromic oxid plus a binder material, such as sodium silicateThe electrodes Il may comprise a wiregauze as illustrated, a preferredmaterial being nickel or platinum. Sheet metal with or withoutperforations may also be employed for the electrodes. A reticulated orforaminated electrode is preferred as it adheres better to the body ofoxide material It.

A preferred method of fabricating these units is as follows: Anelectrode member ll of suitable size is placed on a flat surface and alayer of oxide material applied thereto. The oxide shomely divided andmay be applied as a powder or a paste. The paste has been found to bepreferable. Small amounts of water may be mixed with the powdered oxidet'd'foriii the paste. It is preferable, however, to employ a binder,such as sggium sili cat e It has been found that a cg 1;s m a ig gf ninsulating solid aid ig maintaining a uniform thickness of body T02 Forthis reason, fods lfof glassor like material may be inserted atintervals in the oxide. A second electrode II is then placed over theoxide layer and the assembly pressed sufficiently to bring theelectrodes in contact with the spacers CROSS REFEREiCE it. In units ofthe type illustrated in Figs. 1 and 2 spacer rods of from 3 to 10 milsdiameter have been employed. If desired, the electrodes ll may beflattened to remove major surface irregularities before using. in orderto insure a more uniform thickness of the unit.

If the oxide has been applied as a paste, the units are dried afterassembly. It is not necessary to heat treat the units to obtain apositive temperature coefilcient of resistance. However, heat treatmentdoes aid in producing a more dense, rug ed unit. The temperature of heattreatment may be from 800 to 700 C. but should not go above 700 C.Conductive-leads, such as wires II of nickel, platinum or other suitablematerial, may be welded or otherwise secured to each electrode ll.

Units of the type shown in Figs. 1 and 2 and about 1 centimeter squarewith an electrode spacing of from 8 to 10 mils have a cold resistance offrom 25,000 to 50,000 ohms. The half resistance interval or halftemperature is from 3 to C. within the range O'to 100 C. The halfresistance interval or half temperature may be defined as thetemperature interval over which the resistance doubles or decreases toone-half.

By modification of the foregoing method of fabrication, other units,such as illustrated in Figs. 3, 4, 5 and 6, may be produced. The unitshown in Fig. 3 comprises a body ll of pressed semiconductive materialhaving electrodes ll of pressed metallic powder. In the making of such aunit a layer of powdered chromic oxide material is placed between twolayers of metallic powder, such as permalloy dust (over 30 per centnickel, remainder iron) and subjected to sufficient pressure to form aself-sustaining body. The oxide and metal particles interengage to bondthe electrode portions I l firmly to the body It. Conductive leads llmay be soldered or otherwise secured to the electrodes I I.

In Figs. 4 and 5 are shown units which are made by pressing finelydivided oxidic material into a self-sustaining body II. For the typeshown in Fig. 4, a metallic paste, such as silver paste, is applied totwo portions of body II, and conductors I! are embedded therein.Sufficient heat is applied te convert the paste to metal electrodes llfirmly bonding the conductors it to the unit.

In the units illustrated in Fig. 5, the electrodes H are applied to thebody III by evaporation or plating of metal, such as silver or gold. Theconductors it may then be applied to the electrodes by solder orequivalent means.

In Figs. 3, 4 and 5, the thickness of the conducting layers or lectrodesII has been shown exaggerated in the interest of clarity ofillustration. These electrodes, particularly in the forms shown in Figs.3 and 5, are ordinarily made quite l- As indicated with respect'to theunits illustrated in Figs. 1 and 2, heat treatment is not required toproduce units having a relatively high positive resistance-temperaturecoemcient. However, where heat treatment is desirable from otherconsiderations, such as mechanical strength, the temperature should bekept below 700C. A suitable range is 300 to 700 0.

As illustrated in Fig. 6, units may also be made in bead form. For thistype of unit, the finely divided oxide material is made into a paste,preferably with a sodium silicate binder. The paste is formed into abead 20 on uniformly spaced EXAMINER wires 2|. The wires 2| preferablyare of platinum or other conductive material that will not be affectedby heat treatment. The beads are dried and then preferably given a heattreatment in 5 air. The temperature range as for the cther units may befrom 300 to 700 C. but should not go above 700' C.

As has been stated heretofore, the chromic oxide is preferably mixedwith a bindermaterial, such as sodium silicate. It has been found thatthe binder insures a more rugged unit than is ordinarily possiblewithout a binder. However, the binder is not necessary to obtain apositive resistance-temperature coemcient. Units made with no binder andwith sodium silicate binder up to 25 per cent all have the positiveresistancetemperature coefficient.

. By making suitable circuit changes, units having a high positivetemperature coefiicient of resistance may be employed to perform thesame function as negative resistance-temperature coefilcient devices. Ingeneral, the positive coefllcient unit is connected in series to performthe same function as a negative coefiicient device in parallel or viceversa. For some regulating purposes, a positive coeificient device maybe used with a negative coefiicient device with good results. Anadvantage of the positive coefilcient material is its inherent abilityto prevent the formation of (hot spots," which must be avoided innegative coefilcient units.

A "hot spot," as the name implies, is a portion of a resistor unit thatin use attains a higher temperature than the surrounding portions. Thismay be, for example, due to non-uniformity of the resistance material.In the case of negative resistance-temperature coemcient material, theresistance is reduced at the "hot spot," due to v the high temperature.The resulting greater fiow of current further raises the temperaturethereby further lowering the resistance. In the absence of exteriorcurrent limiting means the hot spot" condition may become so bad as topermanently change the character of the resistance material or evendestroy it. In a positive resistance-temperature coemcient material, thehot spots" are self-limiting since the resistance goes up with increasein temperature.

Although specific embodiments of this inven- 50 tion have been shown anddescribed, it will be understood that they are but illustrative and thatvarious modifications may be made therein without departing from thescope and spirit of this invention as defined in the appended claims.

What is claimed is:

l. A high positive resistance-temperature coefilcient resistor unitcomprising a body of finely divided, pressed chromic oxide heat treatedin air at a temperature below 700' C., and electrodes attached to spacedportions of said body.

2. A resistor having a high positive temperature coefficient ofresistance and comprising a thin body of untreated chromic oxide, twometal- -lic electrodes on opposite faces of said body, and insulatingmeans embedded in said body for maintaining separation between saidelectrodes.

3. A resistor having a high positive temperature coemcientof resistanceand comprising a body of pressed, finely divided, chromic oxide betweentwo metallic electrodes, and a plurality of glass rods embedded in saidbody for maintaining separation between said electrodes.

4. A resistor having ahigh positive temperature coefiicient ofresistance and comprising chromic oxide heat treated at a temperaturebetween room temperature and 700 C.

5. The method 01' making a positive resistancetemperature coemcientresistor unit that comprises forming a body from finely divided chromicoxide, and heat treating said body in air at a temperature below 700 C.

6. The method of making a high positive resistance-temperaturecoefllcient resistor unit that comprises mixing finely divided chiomicoxide and sodium silicate to a paste, applying the paste to areticulated metallic sheet, inserting a plurality of insulating spacermembers into the paste and applying another reticulated metallic sheetthereto, pressing the assembly, drying, and heat

